Friday, August 1, 2014

This photo isn't quite as good an illustration of imbrication as yesterday's, but it's not bad. With the rounder cobbles, you won't see the fabric, but with more tabular (defined in paragraph just above the diagram: large in two dimensions, small in the third) clasts, the upper end will tend to point in the direction the current was moving. Imagine a flat stone coming to rest with its upper end pointing into the current; there's a good chance the current will catch it and flip it over so it points downstream. And in that orientation, the water flow will tend to push it down even more firmly, further stabilizing it. So with imbrication, not every single clast will necessarily point downstream, but there's an easily spotted tendency to do so. A better chance that a randomly chosen cobble will do so, in other words. This shot has more rounded cobbles than yesterday's, and those can't "point" in any direction.

Given that this sediment is Pleistocene in age- quite young and poorly lithified- it's no surprise the flow direction indicated is the same as the modern stream direction. However, this outcrop shows that the imbrication paleocurrent indicator works as advertised, and it can be quite useful in older coarse sediments, where the stream source and destination might not be so obvious or well-known.